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Members of the genus Sphingomonas are important catalysts for removal of polycyclic aromatic hydrocarbons (PAHs) in soil, but their activity can be affected by various stress factors. This study examines the physiological and genome-wide transcription response of the phenanthrene-degrading Sphingomonas sp. strain LH128 in biofilms to solute stress (invoked by 450 mM NaCl solution), either as an acute (4-h) or a chronic (3-day) exposure. The degree of membrane fatty acid saturation was increased as a response to chronic stress. Oxygen consumption in the biofilms and phenanthrene mineralization activities of biofilm cells were, however, not significantly affected after imposing either acute or chronic stress. This finding was in agreement with the transcriptomic data, since genes involved in PAH degradation were not differentially expressed in stressed conditions compared to nonstressed conditions. The transcriptomic data suggest that LH128 adapts to NaCl stress by (i) increasing the expression of genes coping with osmolytic and ionic stress such as biosynthesis of compatible solutes and regulation of ion homeostasis, (ii) increasing the expression of genes involved in general stress response, (iii) changing the expression of general and specific regulatory functions, and (iv) decreasing the expression of protein synthesis such as proteins involved in motility. Differences in gene expression between cells under acute and chronic stress suggest that LH128 goes through changes in genome-wide expression to fully adapt to NaCl stress, without significantly changing phenanthrene degrading activity.

The effect of the inoculant strain Sphingomonas paucimobilis 20006FA on the bacterial composition of a phenanthrene-degrading consortium obtained from a pristine soil in sequencing batch cultures was studied. Inoculated (F200+1) and non-inoculated (F200) phenanthrene-degrading consortia, were obtained. Bacterial diversity of consortia was studied at cultivable (phenotype and genotype characterization) and non-cultivable (PCR-DGGE) levels. During the successive cultures, a loss in the phenanthrene-degrading capacity and a decrease in the bacterial diversity were observed in both consortia. Although inoculation did not produce any significant changes in the consortia phenanthrene-degrading capacity (29.9% F200 and 27.6% F200+1), it did produce changes in the bacterial composition, showing a differential structural dynamics in the DGGE profiles of the inoculated consortium. In both consortia, a dominant band placed at the same position as that of the DNA of the inoculant strain in the DGGE gel could be observed. However, isolated cultures from the consortia which had an identical band position to that of S. paucimobilis 20006FA in the PCR-DGGE profile showed low similarity with respect to the inoculant strain (RAPD).

Polycyclic aromatic hydrocarbons (PAHs) have earned considerable attention due to their widespread environmental distribution and toxicity. In the environment, PAHs decompose by a variety of biotic and abiotic pathways. In both polar and nonpolar environments, phenanthrene (Phe, a common, three-ring PAH) is converted by sunlight to more polar products such as 9,10-phenanthrenequinone (PheQ) and subsequent oxidation products such as the corresponding open-ring dicarboxylic acid product. Biodegradation of phenanthrene also usually leads to oxidative metabolites, and eventually ends in mineralization. Our experimental objective was to investigate the photodegradation of phenanthrene and determine the effect of reaction products such as PheQ on microbial biodegradation of two- and three-ring PAHs. Abiotic experiments were performed to examine the photolytic breakdown of Phe; Phe was converted to PheQ, which catalyzed its own formation. In biodegradation experiments PheQ (0.04-4 mg/L) caused marked inhibition of naphthalene (Nap) biodegradation by a Burkholderia species; Phe did not. Only 20% of the naphthalene was degraded in the presence of PheQ compared with 75% in the control culture with no PheQ added. No PAH-degrading cultures were able to use PheQ as sole carbon source; however, the Phe-degrading enrichment culture dominated by a Sphingomonas species was able to degrade PheQ cometabolically in the presence of Phe. These results may explain why photooxidized phenanthrene-containing mixtures can resist biodegradation.

Bacterium strain EVA17 was isolated from an oil-contaminated soil, and identified as Sphingomonas sp. based on analysis of 16S rDNA sequence, cellular fatty acid composition and physiological-chemical tests. The salicylate hydroxylase and catechol 2, 3-dioxygenase (C230) were detected in cell-free lysates, suggesting a pathway for phenanthrene catabolism via salicylate and catechol. Alignment showed that both of the C230 and GST genes of the strain EVA17 had high similarity with homologues of strains from genus Sphingomonas. The phylogenetic analysis based on 16S rDNA and C230 gene sequence indicated that EVA17 should be classified into genus Sphingomonas, although the two phylogenetic trees were slightly different from each other. The results of coamplification and sequence determination indicated that GST gene should be located upstream of the C230 gene.